Friction stir welding using a superabrasive tool

ABSTRACT

A probe for friction stir welding MMCs, ferrous alloys, non-ferrous alloys, and superalloys, as well as non-ferrous alloys, the probe including a shank, a shoulder, and a pin disposed through the shoulder and into the shank, wherein the pin and the shoulder at least include a coating comprised of a superabrasive material, the pin and shoulder being designed to reduce stress risers, disposing a collar around a portion of the shoulder and the shank to thereby prevent movement of the shoulder relative to the shank, and incorporating thermal management by providing a thermal flow barrier between the shoulder and the shank, and between the collar and the tool.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/658,919, filed Feb. 16, 2010, now U.S. Pat. No. 7,993,575, which is acontinuation of U.S. patent application Ser. No. 11/637,606, filed Dec.12, 2006, now U.S. Pat. No. 7,661,572, which is a continuation of U.S.patent application Ser. No. 10/705,717, filed Nov. 10, 2003, now U.S.Pat. No. 7,152,776, which is a continuation of U.S. patent applicationSer. No. 09/851,596, filed May 8, 2001, now U.S. Pat. No. 6,648,206,which claims priority to U.S. Provisional Patent Application Ser. No.60/202,665, filed May 8, 2000.

BACKGROUND

1. The Field of the Invention

This invention relates generally to friction stir welding wherein heatfor creating a weld is generated by plunging a rotating pin of a toolinto a workpiece. More specifically, the present invention relates to anew tool that is used in a friction stir welding process that enablesthe present invention to weld materials that are not functionallyweldable using state of the art friction stir welding processes andtools, said materials including ferrous alloys such as stainless steel,and higher melting point super alloys that contain only small amounts ofor no ferrous materials at all.

2. Background of the Invention

Friction welding has been used in industry for years. It is asolid-state process that yields large economic benefits because itavoids many problems associated with rapid solidification of moltenmaterial that occurs in traditional fusion welding processes.

One example of friction welding occurs when the ends of two pipes arepressed together while one pipe is rigidly held in place, and the otheris pressed against it and turned. As heat is generated by friction, theends of the pipes become plasticized. By quickly stopping rotation ofthe pipes, the two pipes fuse together. Note that in this case, thefrictional heating is caused by the relative motion of the two parts tobe joined.

In contrast, FIG. 1 is a perspective view of a tool being used forfriction stir butt welding that is characterized by a generallycylindrical tool 10 having a shoulder 12 and a pin 14 extending outwardfrom the shoulder. The pin 14 is rotated against a workpiece 16 untilsufficient heat is generated, wherein the pin of the tool is plungedinto the plasticized workpiece material. The workpiece 16 is often twosheets or plates of material that are butted together at a joint line18. The pin 14 is plunged into the workpiece 16 at the joint line 18.The frictional heat caused by rotational motion of the pin 14 againstthe workpiece material 16 causes the workpiece material to softenwithout reaching a melting point. The tool 10 is moved transverselyalong the joint line 18, thereby creating a weld as the plasticizedmaterial flows around the pin from a leading edge to a trailing edge.The result is a solid phase bond 20 at the joint line 18 that isgenerally indistinguishable from the workpiece material 16.

The prior art is replete with friction stir welding patents that teachthe benefits of using the technique to obtain welds that have beneficialcharacteristics over contemporary fusion welding processes. Thesebenefits include low distortion in long welds, no fumes, no porosity, nosplatter, and excellent mechanical properties regarding tensilestrength. Furthermore, the process has the advantage of using anon-consumable tool, no need for filler wire, no need for gas shielding,and a tolerance for imperfect weld preparations such as the presence ofoxide in the weld region. The process is especially useful forpreventing significant heat damage or otherwise altering the propertiesof the original material being welded.

However, it has long been a desire of industry to be able to weldmaterials that are presently functionally unweldable for friction stirwelding. Thus, while friction stir welding is a very advantageoustechnique for welding non-ferrous alloys such as aluminum, brass andbronze, there has been no tool that is capable of functionally weldingmaterials having higher melting points. It should be understood thatfunctionally weldable materials are those that are weldable usingfriction stir welding in more than nominal lengths, and withoutdestroying the tool.

Unfortunately, fusion welding alters or damages the alloy at the weld,thereby compromising the weld as a result of the defects or adversephases which form in the weld during the welding process. In some cases,the non-metallic reinforcement material which has been joined with theoriginal workpiece material to create the alloy is depleted at the weld.The result is a weld that has properties and characteristics which aredifferent from the unaltered areas of the original workpiece material.

Until now, it has been the nature of friction stir welding that using aconventional friction stir welding tool or probe is worn downsignificantly so as to prevent functional welding of materials such asMMCs, ferrous alloys, and superalloys. Most tools simply do not work atall in MMCs, ferrous alloys, and superalloys. If a conventional toolcould begin friction stir welding, the wear would be so significant thata probe would be torn apart after only a short distance. For example,some alloys will cause wear on a probe such that it can no longerfunction after welding for a distance of only inches.

Unfortunately, it is generally the case that it is not possible tosimply insert a new tool and begin the friction stir welding processwhere the previous probe failed. If the weld is not continuous anduninterrupted, it is useless because of mechanical weakness.Furthermore, a portion of the tool is typically left behind in theworkpiece material, also contributing to the mechanical weakness.

Therefore, it would be an advantage over the prior art to provide a newtool for use with the friction stir welding process that enables longercontinuous and uninterrupted welding runs (functional welding) ofmaterials that will cause a conventional tool to fail after a shortdistance. It would also be an advantage over the prior art if the newtool made it possible to friction stir weld materials that werepreviously too difficult to weld with conventional friction stir weldingtools. It would also be an advantage to provide a tool that would enablefriction stir welding with conventional workpiece materials, whileexhibiting improved wear characteristics for the tool.

A first class of materials that would be desirable to friction stir weldbut are functionally unweldable with conventional tools are known asmetal matrix composites (MMCs). An MMC is a material having a metalphase and a ceramic phase. Examples of the ceramic phase include siliconcarbide and boron carbide. A common metal used in MMCs is aluminum.

MMCs have desirable stiffness and wear characteristics, but they alsohave a low fracture toughness, thereby limiting applications. A goodexample of a use for MMCs is in disk brake rotors on vehicles, wherestiffness, strength and wear provide advantages over present materials,and where the more brittle nature is generally not an issue. The MMCmakes the rotor lighter than cast-iron, and the ceramic phase such assilicon carbide enables greater wear resistance.

Other important applications for MMCs include, but should no beconsidered limited to, drive shafts, cylinder liners, engine connectingrods, aircraft landing gear, aircraft engine components, bicycle frames,golf clubs, radiation shielding components, satellites, and aeronauticalstructures.

A second class of materials that would be desirable to friction stirweld, and which have much broader industrial applications, are ferrousalloys. Ferrous alloys include steel and stainless steel. Possibleapplications are far-ranging, and include the shipbuilding, aerospace,railway, construction and transportation industries. The stainless steelmarket alone is at least five times greater than the market for aluminumalloys. It has been determined that steels and stainless steelsrepresent more than 80% of welded products, making the ability tofriction stir weld highly desirable.

Finally, a third class of materials that would be desirable to frictionstir weld, have broad industrial applications, have a higher meltingpoint than ferrous alloys, and either have a small amount of iron ornone, are the super alloys. Superalloys are nickel-, iron-nickel, andcobalt-base alloys generally used at temperatures above 1000 degrees F.Additional elements commonly found in superalloys include, but are notlimited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium,tantalum, and rhenium.

It is noted that titanium is also a desirable material to friction stirweld. Titanium is a non-ferrous material, but has a higher melting pointthan other non-ferrous materials.

There are significant challenges that have so far prevented the creationof a tool that can functionally weld MMCs, ferrous alloys, andsuperalloys. Some of these challenges only became apparent duringexperimentation as the inventors initially attempted to modify existingtools that can friction stir weld non-ferrous alloys. These challengesand the evolution of the tool will be discussed so as to enable thereader to practice the invention.

SUMMARY OF INVENTION

It is an object of the present invention to provide a new tool for usein friction stir welding that has improved wear characteristics overconventional tools.

It is another object to provide the new tool that includes asuperabrasive material that enables friction stir welding of MMCs,ferrous alloys, and superalloys, as well as non-ferrous alloys.

It is another object to provide the new tool that enables improved weldcharacteristics for the non-ferrous alloys.

It is another object to provide the new tool that enables finishingcosts of a welded material to be reduced.

It is another object to provide the new tool that has an improvedgeometry to reduce wear of the tool when friction stir welding MMCs,ferrous alloys and superalloys.

It is another object to reduce thermal, mechanical and chemical wear ofthe new tool.

It is another object to provide thermal management for the new tool.

In a preferred embodiment, the present invention is a tool for frictionstir welding MMCs, ferrous alloys, non-ferrous alloys, and superalloys,the tool including a shank, a shoulder, and a pin disposed through theshoulder and into the shank, wherein the pin and the shoulder at leastinclude a coating comprised of a superabrasive material, the pin andshoulder being designed to reduce stress risers, disposing a collararound a portion of the shoulder and the shank to thereby inhibitrotational movement of the shoulder relative to the shank, andincorporating thermal management by providing a thermal flow barrierbetween the shoulder and the shank, and between the collar and the tool.

In a first aspect of the invention, the shank, shoulder, and pin areseparate components that are coupled together to form the friction stirwelding tool, wherein the shoulder and the shank include a superabrasivecoating.

In a second aspect of the invention, the shank and the shoulder are amonolithic element including a superabrasive coating over at least aportion thereof, and having a separate pin with a superabrasive coating.

In a third aspect of the invention, the shank, shoulder and pin are amonolithic element having a superabrasive coating covering at least aportion thereof.

In a fourth aspect of the invention, thermal management of heat usingthermal flow barriers within the tool enables sufficient heat to begenerated at the pin to enable friction stir welding, while protecting atool holder from heat damage.

In a fifth aspect of the invention, stress risers are reduced on thepin, larger radii are provided on the shoulder, and pin diameter isincreased to thereby enable friction stir welding of MMCs, ferrousalloys, and superalloys.

In a sixth aspect of the invention, the tool includes at least one CVD,ion-beam implanted, and/or PVD coating disposed over the superabrasivecoating to thereby increase resistance to chemical and mechanical wear.

In a seventh aspect of the invention, the tool is coated with a whiskerreinforced superabrasive in order to decrease spalling of thesuperabrasive coating.

In an eighth aspect of the invention, flats are disposed along thelengthwise axis of the tool to thereby prevent separation of the toolinto component elements during translational motion of the tool.

In a ninth aspect of the invention, the superabrasive coating isselected based upon a desired balance between chemical wear andmechanical wear.

In a tenth aspect of the invention, the superabrasive coating isselected based upon the characteristic of having a low coefficient offriction that prevents the workpiece material from adhering to the tool,thereby reducing wear of the tool.

These and other objects, features, advantages and alternative aspects ofthe present invention will become apparent to those skilled in the artfrom a consideration of the following detailed description taken incombination with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a state of the art friction stir weldingtool that is welding two plates of material together.

FIG. 2A is a cross-sectional profile view of the preferred embodiment,made in accordance with the principles of the present invention.

FIG. 2B is an end view of the tool shown in FIG. 2A.

FIG. 3 is a cross-sectional view of an alternative embodiment, where theshank, shoulder and pin are all separate components.

FIG. 4 is a cross-sectional view of another alternative tool embodiment,where a hole is disposed through the length of the shank to assist inpin replacement.

FIG. 5 is a cross-sectional view of another alternative tool embodiment,where the shank, shoulder and pin are a monolithic element.

FIG. 6A is a cross-sectional view of an endmill blank that isfunctioning as a pin, the pin having helical channels in which isdisposed superabrasive material.

FIG. 6B is an end view of the endmill blank of FIG. 6A.

FIG. 7A is a cross-sectional view of another alternative toolembodiment, where the shank also functions as a locking collar.

FIG. 7B is a close-up profile view of surface irregularities thatenables mechanical locking to thereby prevent slipping of the shoulderrelative to the shank.

FIG. 7C is a close-up profile view of surface irregularities thatenables mechanical locking to thereby prevent slipping of the shoulderrelative to the shank.

FIG. 8 is a cross-sectional profile view of a pin having surfacedeformations designed to create transitional or turbulent flow aroundthe pin in the workpiece material.

FIG. 9A is an end view of a pin that includes surface deformations inthe form of a flat on the pin designed to create transitional orturbulent flow around the pin.

FIG. 9B is an end view of a pin that includes surface deformations inthe form of an irregular surface on the pin designed to createtransitional or turbulent flow around the pin.

FIG. 10 is cross-sectional profile view of a tool that has an off centerpin or cam designed to create transitional or turbulent flow around thepin.

DETAILED DESCRIPTION

Reference will now be made to the drawings in which the various elementsof the present invention will be given numerical designations and inwhich the invention will be discussed so as to enable one skilled in theart to make and use the invention. It is to be understood that thefollowing description is only exemplary of the principles of the presentinvention, and should not be viewed as narrowing the claims whichfollow.

The presently preferred embodiment of the invention is a tool thatincorporates superabrasive materials in a pin and shoulder, and utilizesthermal management within the tool, to enable friction stir welding ofmaterials that are presently functionally unweldable. Thus, the presentinvention makes possible long, continuous, and uninterrupted welds ofMMCs, ferrous alloys, and superalloys without suffering significantdegradation of the tool.

The development of the presently preferred embodiment presentedsignificant challenges because conventional tools wore out or broke whenused on MMCs, ferrous alloys, and superalloys. These challenges can besummarized as thermal wear, mechanical wear, chemical wear, thermalmanagement and geometry of the tool. The solutions to these challengesposed significant problems until the materials selected for the toolwere combined with the correct tool geometry and thermal management, aswill be explained through illustration of various embodiments of theinvention.

The sequence of events that culminated in the tool of the presentinvention that is capable of functionally welding MMCs, ferrous alloys,and superalloys began with a test on a broken tool. This broken tool hada pin broken off while running a weld on an workpiece material made fromMMC. Therefore, only a raised shoulder formed of CBN was disposedthereon, with a locking collar. The inventors never expected theshoulder to withstand the chemical or mechanical wear on a ferrousmaterial, but wondered what would happen. Surprisingly, the shouldershowed no significant signs of wear after a long and continuous run overa workpiece having a high melting point.

The success of the test led the inventors to experiment with varioustool embodiments, trying to identify those characteristics of the toolthat could take advantage of the surprising wear and thermal resistanceresults of the broken tool.

FIG. 2A is a cross-sectional profile view of the elements of thepreferred embodiment of the present invention that is a result of thosetests. Beginning with the mechanical elements, the tool 28 includes ashank 30 that is generally cylindrical. Coupled to the shank 30 is ashoulder 32 with an integral pin 34. Coupled around a portion of theshank 30 and the shoulder 32 with an integral pin 34 is a collar 36.Disposed between the shank 30 and the shoulder 32 with an integral pin34 is a thermal flow barrier 38. There is also a thermal flow barrier 40disposed between the collar 36 and a portion of the shank 30, as well asthe shoulder 32 with an integral pin 34.

This preferred embodiment incorporates several novel elements, only someof which are readily apparent from FIG. 2A. First, the preferredmaterials used in construction of the tool 30 are critical to theinvention. The shank 30 is preferably cemented tungsten carbide.Cemented tungsten carbide is selected for its strength, and for its highthermal conductivity that allows proper cooling of the shank to maintainits strength relative to the other materials used in the tool 28.

One advantageous characteristic of the superabrasive material is itshigh thermal conductivity. However, it is important to understand thatthe thermal conductivity can be useful or a detriment to the tool,depending upon how it is managed. Experimentation has demonstrated thatthermal management is critical to creating a successful friction stirwelding tool.

For example, when the pin 34 is coated with a superabrasive material,this resulted in significant amounts of heat being drawn away from theweld region. To compensate, the tool 28 had to be driven harder thandesired to create more heat in the weld region. Therefore, a successfultool has to direct sufficient heat to the weld region to enablesolid-phase welding, while at the same time limiting the heat so thatthe weld region is kept as cool as possible in order to obtain a highquality weld region. In other words, with high thermal conductivity ofthe superabrasive, the tool can be designed to regulate any desired flowof heat out of the tool, thereby enabling design flexibility. Incontrast, a material with lower thermal conductivity would be limited toits own value of thermal conductivity, or less.

Where a lot of heat is generated by the tool because of required runparameters, it may be necessary to resort to external cooling of thetool. The objective is to have a tool whose thermal flow characteristicscan be modified in order to obtain the best weld characteristics,including a weld that cools fast.

A thermal management scheme was developed in order to maintain the heatgenerated by friction between the tool and the workpiece near the weldregion. One aspect of the scheme is to select a material for the shank30 that will restrict heat flow from the pin 34, to a tool holder (notshown) that is gripping the attachment end 42 of the tool 28. The toolholder causes the tool 28 to rotate, and it might also be damaged byheat. The thermal management scheme also keeps the shank cool enough toresist translational forces during friction stir welding.

Alternatively, a high strength steel can be substituted for the cementedtungsten carbide in the shank 30, but the steel will conduct lessthermal energy away from the shoulder 32 and integral pin 34, therebycausing the shank to run at a higher temperature and reduced strength.However, the steel will function with the proper cooling.

If rotating the pin 34 against the workpiece material is what enablesheat to be generated for the friction stir welding process, it isimportant to know what rates of rotation will result in a functionalweld. The rate of rotation of the shoulder 32 with an integral pin 34 ispreferably within the range of 50 rpm to 2000 rpm, depending upon thematerial being welded, the diameter of the tool, and the composition ofthe elements of the tool 28. It is noted that the preferred surfacespeed of the tool is between 7 and 400 surface feet per minute.

Along with the rate of rotation, the timing of the friction stir weldingprocess is not trivial either. It is important that the pin 34 beplunged into the workpiece material only when it is sufficiently heatedto form a plasticized welding region. However, this timing changes forthe materials being used, for each tool configuration, and for theprocess parameters used.

The purpose of the thermal flow barrier 38 can now be understood inlight of the previous comments regarding thermal flow management. It iscritical that the frictional heat be properly managed to keep heatfocused on the workpiece material without drawing it away through thetool 28. In the presently preferred embodiment, titanium or a titaniumalloy is selected as the material for the thermal flow barrier 38. Atitanium alloy is selected because of its ability to withstand thetemperatures that are experienced by the tool 28, and because of itsrelatively low thermal conductivity. Nevertheless, it should be realizedthat the titanium alloy is not the only material that can be used. It isdescribed for illustration purposes, and can be replaced with a materialthat performs a similar function.

The shoulder 32 with integral pin 34 is a most novel element of theinvention because of the materials used in its fabrication, and becauseof its geometry. These elements are selected in order to overcome theextreme thermal, mechanical, and chemical wear of the friction stirwelding process. One type of wear is not necessarily more important thananother, they just result in different types of failures.

Regarding material, it has been determined through experimentation thatusing a superabrasive on the shoulder 32 and the pin 34 has enabled theinvention to achieve functional welding of MMCs, ferrous alloys,non-ferrous alloys, and superalloys. Specifically in the preferredembodiment, polycrystalline cubic boron nitride (PCBN) is used as asuperabrasive-coating on a substrate material being used for theshoulder 32 with the integral pin 34.

To state that a coating of PCBN is utilized as a superabrasive coatingon the substrate material might imply that the application process istrivial. This is far from the case. The coating is not merely asubstance that is wiped on using a room temperature process. Rather, theapplication involves high temperatures and ultra high pressures.Furthermore, the geometry of the surface to which the superabrasive isapplied has much to do with the ability of the superabrasive to wearwell, and to avoid failure from cracking. Accordingly, an importantaspect of the invention is to describe a tool geometry that will obtainthe best results from a superabrasive coating. Another important aspectof the invention is to describe the possible superabrasive materialsthat can be used. One example of how the coating is applied will now beprovided.

PCBN is made from hexagonal boron nitride power in an ultra hightemperature and ultra high pressure (UHTP) press (one million psi at1400 degrees Celsius, or 1673 K). Time and temperature are adjustable tocreate cubic boron nitride crystals having the optimum size, shape andfriability for specific applications. The crystals range in size ofdiameter from under a micron to around 50 microns.

For fabricating the shoulder 32 and integral pin 34 of the presentinvention, the cubic boron nitride (CBN) crystals are mixed with apowder of a different or second phase material. The second phasematerial is either ceramic or metal based. The CBN provides mechanicalstrength, while a ceramic would provide resistance to chemical wear.Therefore, the percentage of CBN relative to the second phase materialis dependent upon the application, where a balance must be struckbetween mechanical and chemical wear resistance.

It has been determined that the second phase material generally adds atoughness and chemical stability to the PCBN. The toughness is in partdue to the ability of the second phase to inhibit crack propagation. TheCBN helps here as well, as it has randomly oriented fracture plans thatnaturally resist spalling. Lower CBN content is generally used formachining operations of hardened high temperature superalloys needingmore chemical wear resistance and less mechanical wear resistance.Higher CBN content is used for abrasive wear resistance, where thesecond phase is generally metallic for added toughness.

It is important to note that CBN crystals have hardness values, thermalconductivity, thermal expansion, coefficient of friction values,fracture toughness, and transverse rupture values similar to diamond.These properties are engineered using the second phase material toachieve a specific application requirement.

The mixed powder is placed with a substrate such as cemented tungstencarbide, or even a free-standing PCBN blank, in a refractory metalcontainer. The container is sealed and returned to the UHTP press, wherethe powder is sintered together and to the substrate to form a PCBN toolblank. The PCBN tool blank is then either ground, lapped, wire EDM cut,or laser cut to shape and size, depending upon the application.

Superabrasives are materials that are defined as being processed underhigh temperature and ultra high pressure. Superabrasive materialsinclude PCBN and polycrystalline diamond (PCD). These materials aregoing to be found on the periodic table and identified as compoundsincluding elements extending from IIIA, IVA, VA, VIA, IIIB, IVB, and VB.

Superabrasives have a hard primary or first phase, and a secondarycatalytic phase that facilitates primary phase crystal structuresintering and transformation. Superabrasives may or may not beelectrically conductive. They can also be strengthened using whiskerreinforcement. They may also be considered as materials that undergo asolid-state phase transformation during processing at elevatedtemperature and pressure, and a material that is created by a sinteringprocess, with or without a binder.

Another aspect of the invention concerns the shoulder 32. Depending uponhow it is manufactured, the superabrasive material on the shoulder 32may be relatively thin. This becomes important if the superabrasivematerial is being finished to a desired form. If the finished formincludes a slanted, beveled or angled surface or other similar structureas shown in this embodiment, it is important that the slant not piercethe superabrasive material. Accordingly, the thickness of thesuperabrasive must be sufficient to provide the desired slant withoutreaching the substrate material.

As the present invention was being developed, a tool designed forfriction stir welding of aluminum was used on a ferrous alloy. The toolfailed at the pin. This is because there are geometrical considerationsthat must be taken into account when friction stir welding hardermaterials with higher melting points, and when using a shoulder and pinthat are coated with a superabrasive material. Thus, other novelfeatures of the invention include 1) the elimination of stress risers,2) the use of larger radii or chamfers, 3) more uniform distribution ofstresses, and 4) increasing the diameter of the pin.

Regarding stress risers, many prior art patents teach exotic pindesigns, including threaded pins, and pins having sharp edges andangles. Screw threads on a pin are generally desirable because thethreads push the workpiece material back down into the workpiece causinga stirring action and a better weld. However, these shapes are generallyundesirable in the present invention because they function as crackinitiators for a superabrasive coating, but can be used in a modifiedform to minimize the stress riser. Therefore, large radii or chamfers onthe shoulder 32 and the pin 34 are desirable. These large radii areshown in FIG. 2A.

Regarding pin diameter, the pin 34 of the preferred embodiment is largerin diameter than conventional tools. This is partly due to the greaterstresses that the pin 34 will experience when friction stir weldingMMCs, ferrous alloys, and superalloys. The pin diameter is probably bestexpressed as a ratio of pin diameter compared to pin length. In thepresently preferred embodiment, the range of ratios extends from 0.2:1to 30:1.

It is also noted that the shoulder 32 is not shown as a flat surfacerelative to a workpiece. The shoulder 32 is in fact concave. This shapeenables the plasticized workpiece material to be more easily displacedand flow around the pin 34. The concave shape also forces theplasticized workpiece material back into the workpiece.

Although a relatively flat region 44 is shown between the outer andinner radii of the shoulder 32, this region 44 can also be curved toform a concave or a convex surface. Alternatively, the shoulder 32 canbe also be convex or flat relative to the workpiece.

The friction stir welding process requires that the tool holder pressdown on the tool 28. This axial pressure is generally sufficient to holdthe components 30, 32, 34 together in the axial direction. However, asthe tool 28 is translationally moved with respect to the workpiece,significant rotational forces are urging the shank 30 to move relativeto the shoulder 32. It is critical that the elements not turn-relativeto each other. Relative rotational movement would prevent the pin fromgenerating sufficient frictional heat against the weld region of theworkpiece.

Therefore, it is a novel element of the invention to require that theelements be mechanically locked. Mechanical locking is necessary becausebrazing the components together will only serve to function as a pointof weakness of the tool 28. This is because the brazing material islikely to have a melting point that is at most near the temperature atwhich friction stir welding is being performed.

Mechanical locking was first accomplished via dovetailing in earlyexperiments. However, the dovetails propagated crack formation inportions of the tool 28. Therefore, it is preferred to use flats 46 asshown in FIG. 2B. The flats 46 prevent slipping of the tool 28components 30, 32, 34 relative to each other, combination with a lockingcollar. Although the figure shows two flats, 1 to any desired number canbe disposed around the circumference of the tool 28.

Alternatively, the flats 46 can be replaced by other surface featuresthat enable the tool components 30, 32, 34 to be mechanically lockedinto a position where they will remain stationary relative to eachother. The other surface features include the use of splines, frictionwelding, diffusion welding, lock on back, or a lock on the outsidediameter of the shank.

The final components of this preferred embodiment are the collar 36 andthe thermal flow barrier 40. One of first collar materials that was usedin experimentation was formed of a titanium alloy. Disadvantageously,titanium alloy is drawable, and will creep and flow under hightemperatures. Initial tests with a titanium collar showed that thetitanium alloy collar actually fell down around the shoulder and pin andonto the workpiece as the tool made a welding run.

Accordingly, it was decided that another material would be fastenedaround the shoulder 32 and the shank 30 to mechanically lock themtogether. In addition, the titanium alloy would still be present inorder to insulate the new collar material from the high temperatures ofthe shoulder 32 and pin 34. This insulation also assisted in thermalmanagement to thereby maintain the desired temperature at the weldingregion of the workpiece. The presently preferred embodiment utilizes asuperalloy for the material of the collar 36. For example,nickel-cobalt, or cobalt-chromium are suitable superalloy materials.

FIG. 2B is provided as an end-view of the tool 28. The materials thatare visible from this perspective are the pin 34, the shoulder 32, thetitanium alloy thermal flow barrier 40, and the collar 36.

Although the preferred embodiment teaches the use of CBN as thesuperabrasive coating on the shoulder 32 and the pin 34, this is not theonly superabrasive material that can be used. For example, one of thebest substitutes for CBN is polycrystalline diamond. It is well knownthat PCD exhibits many of the performance characteristics of CBN.

Dimensions of the preferred embodiment are only useful as an example,but will be provided. The diameter of the pin is 0.37″. The diameter ofthe shoulder is 1″. The thickness of the titanium alloy thermal barriers38, 40 are 0.060″, and the diameter of the collar 36 is 1.63″. The angleon the collar 36 is shown as 15 degrees, and the angle of the shoulderis shown as 6 degrees. These figures are for illustration purposes only,and should not be considered limiting. Nor will these dimensions workfor all applications.

There are various issues that need to be explained in order tounderstand all of the advantages and requirements of the presentinvention, and how the preferred embodiment was developed.

One important consideration of the tool 28 is that while CBN is a goodmaterial for friction stir welding steel, it may not be good for othermaterials. Therefore, it is an element of the invention to make itpossible to mix and match shoulders and pins, as will be shown inalternative embodiments.

Another consideration is that some superabrasives are soluble in certainmaterials. For example, PCD has a chemical reaction with a titaniumalloy at friction stir welding temperatures. Thus, diamond cannot beused to weld materials that are carbide formers, unless the highesttemperature that will be reaches is below a soluble point.

There are two distinct advantages to using superabrasives in theshoulder 32 and the pin 34 of the present invention. First of all, thecoefficient of friction of CBN and of diamond is very low (0.05 to 0.1).In contrast, the coefficient of friction of steel is 0.8. This lowcoefficient of friction enables the workpiece material to slide alongthe tool 28 instead of sticking to it. The result is a much cleanerfinish that does not require a lot of finishing work. Finishing costscan be high, especially with ferrous alloys and superalloys. The lowcoefficient of friction also leads to reduced tool wear.

Second, the thermal conductivity of CBN and PCD are high, about 100 to550 Watts/meter-K, compared to steel which is about 48 Watts/meter-K.The result is that the weld is cooler. Cooler welds are desirablebecause they form further away from the melting point, and thus avoidall of the problems of liquid welding phases. It has been demonstratedin tests that one of the direct benefits of the present invention isthat the welds have greater tensile strength compared to welds usingmore conventional arc welding. Of course, the high thermal conductivityof CBN is also the reason for the user of thermal flow barriers 38, 40that are used to keep the heat from escaping the weld region of theworkpiece.

It has been explained that a substrate for the shoulder and the pin hasbeen coated with a superabrasive. It is another novel element of theinvention to allow for multiple coatings. These coatings can be appliedusing CVD, ion-implantation, or PVD processes. The purpose of thecoatings is to provide features that will assist the superabrasive towithstand the different types of wear that it experiences. For example,a second coating can enhance the chemical wear resistance.

The coatings that are applied to substrates or on top of thesuperabrasives can be of varying thicknesses. Although in the abrasivetool industry a coating of 0.030″ to 0.050″ is considered a thickcoating, and a coating of less than 0.001 is considered a thin coating,it is an aspect of the invention that other thicknesses may be requiredfor optimum performance of the coating material. Solid CBN can also bepressed so that it has no coating. This CBN can be pressed to as large avolume as the UHTP process will allow, usually up to 4 inches indiameter by 4 inches long. This solid CBN does not, however, have thebenefit of a substrate that adds strength and toughness.

While thermal management is a novel element of the preferred embodiment,cooling of the tool is also important, but for a different reason.Thermal management is used to ensure that enough heat is directed to theweld region by making sure it is not siphoned away. But for the heatthat is able to move away from the shoulder and pin, it is oftennecessary to provide some type of active cooling. Cooling can take theform of a mist directed at the exterior of the tool, or even air. Butcooling can also be an internal process. Thus, it may be necessary withsome materials to provide internal cooling by providing a coolingchannel through a portion of the shank. It is also possible to cool thetool holder. Cooling can even extend to the workpiece itself. While heatis necessary for the weld, it should always be remembered that a coolweld is inherently stronger, and that friction stir welding is asolid-state process.

There are several alternative embodiments that must also be consideredin order to understand performance issues. The presently preferredembodiment teaches a tool having two components, the shank 30, and theshoulder 32 with an integral pin 34. However, experimentation has shownthat the pin 34 will usually wear out before the shoulder 32. If theyare integral, the entire shoulder 32 and pin 34 combination have to bereplaced together. This is a waste of resources.

Therefore, FIG. 3 is a cross-sectional perspective view of analternative embodiment wherein the shoulder 50 is not integral with thepin 52. Instead, these components are manufactured separately, andcoupled to the shank 48. As shown in FIG. 3, the pin 52 rests within abore hole 54 drilled into an end of the shank 48. The thermal flowbarriers 38, 40 are still in place, except for where the pin 52 extendsinto the shank 48. However, if the pin 52 only has a superabrasivecoating on the portion that is outside of the bore hole 54, then thecemented tungsten carbide (or other suitable) substrate of the pin 52 isnot going to be as good a thermal conductor to the shank 48.Nevertheless, in an alternative embodiment, the thermal barrier can beextended down into the bore hole 54.

Coupling the pin 52 to the shank 48 is also not a trivial matter.Preferably, the pin 52 is disposed into the bore hole 54 using a pressfitting. However, it is likely that a hex or square shaped hole may bedesirable. It is noted that it is possible to add strength to the pin 52if it is put into residual compressive stress.

Residual compressive strength can be created, for example, by heatingthe tool. As the tool is heated, it expands. The diameter of the pin isselected so that when the tool cools, it exerts positive mechanicalpressure on the pin.

Another method of attachment might be to dispose a screw into the pin52. The screw would be used to pull the pin into compression through theback end of a tool.

FIG. 4 is provided as another alternative embodiment of the presentinvention. The difference from FIG. 3 is that the hole 62 now extendsentirely through the shank 60. One of the main advantages of thisembodiment is that replacing the pin 64 is simply a matter of pushingthe pin 64 out of the shank 60 by inserting a tool through the hole 62.This design can reduce costs, and make the tool reusable for manyapplications. It is thus only necessary to insert a pin 64 of the properlength.

One aspect of the invention that has not been addressed is thecomposition of the pin when it is a replaceable item. Preferably, thepin is manufactured from cemented tungsten carbide, and coated with anappropriate superabrasive. However, the pin can also be manufactured asa solid superabrasive material, or be a carbide with the desiredcoating.

Another aspect of the pin is that it can be reinforced. Reinforcing apin may be desirable if the pin length is unusually long because of thethickness of the workpiece material. Reinforcement may also be necessarywhen the material of the pin does not inherently have the strength of amaterial such as tungsten carbide.

In a similar manner, the shank can also be manufactured from asuperabrasive, or be a carbide that is coated with a superabrasive.

FIG. 5 is another alternative embodiment of the invention, whereininstead of having separate components, the tool 70 is a monolithic unit.However, the cost of manufacturing an entire tool as a single piece isprohibitively expensive. Given the advantages of the other embodiments,it is unlikely that this embodiment will be widely used. Nevertheless,it is an option that would likely be formed from cemented tungstencarbide, with a superabrasive coating applied to the shoulder 72 and pin74 areas. The difficulty in its use might be the thermal management thatis seen as critical when using superabrasives. Therefore, insertion of athermal barrier may be important, but that would defeat the purpose ofthe monolithic design.

An alternative embodiment of the invention is the type of pin that isinserted into the tool. FIG. 6A is provided as a profile perspectiveview of a helical endmill blank 80. The substrate of the blank 80 ispreferably cemented tungsten carbide, with the PCBN or othersuperabrasive disposed in helical channels 82. FIG. 6B is an end view ofthe blank 80, illustrating the helical channels 82 in which thesuperabrasive material is disposed.

It is envisioned that there will be other means for coupling a PCBNshoulder or coated shoulder to a shank. FIG. 7A is provided as across-sectional illustration of another tool embodiment that providesall of the desirable characteristics of the present invention, butwithout the use of a separate locking collar. In this figure, thelocking collar is replaced by a portion of the shank itself so that itis integral to the shank itself.

Specifically, the shank 90 is shown having a bore hole 92 disposedpartially into the working end 94 of the shank. The depth of the borehole 92 is selected based upon the depth of the shoulder and the pin 96.In this embodiment, the shoulder and the pin 96 are integral. However,the shoulder and the pin 96 could also be separate components as shownin previous embodiments. What is important about this embodiment is thatthe wall 98 around the bore hole 92 functions as a locking collar, tothereby assist in preventing rotational movement of the shoulder and thepin 96 relative to the shank 90. This can be accomplished, for example,by press fitting the shoulder and the pin 96 into the bore hole 92.Notice that the thermal flow barrier 100 is also in place to enablemanagement of heat from the shoulder and the pin 96 to the shank 90.

However, it is possible that under some load conditions, the shoulderand the pin 96 may slip despite the press fitting. Therefore, it is alsoenvisioned that this embodiment includes the use of some other means formechanically locking the back surface 102 of the shoulder and the pin 96to the shank 90. This can be accomplished using some of the previouslymentioned techniques. For example, mechanical locking can be performedby complementary dentations, splines or other physical features on theback surface 102 and the bottom surface 104 of the bore hole 92 thatprevent relative rotational movement through complementary interlocking.

FIGS. 7B and 7C are provided as an illustration of just two examples ofhow the back surface 102 of the shoulder and the pin 96 can bemechanically locked to the bottom surface 104 of the bore hole 92 of theshank 90. In this figure, bore hole splines 106 are formed in the bottomsurface 104 of the bore hole 92, and complementary splines 108 areformed on the back surface 102 of the shoulder and the pin 96.

An important and novel aspect of the invention also pertains to the flowof the workpiece material around the tool pin. Although friction stirwelding is said to occur as a solid-phase process, the workpiecematerial is still capable of fluid-like movement, or flow. It isimportant when trying to obtain the best weld possible to increase therate of flow of the workpiece material around the pin.

Laminar flow is defined as non-turbulent fluid flow. Unfortunately,laminar flow of the workpiece material is also the slowest. Therefore,any geometry of the pin that will result in the increased rate of fluidflow of the workpiece material around the pin will also result in a weldhaving the improved weld characteristics. Accordingly, it is desired tohave a turbulent fluid flow of the workpiece material, or transitionalflow which is defined as a flow that has turbulent characteristics.Therefore, it is desirable to trip a boundary layer from laminar flowinto the transitional or turbulent type of flow. Furthermore, it is alsodesirable to obtain the transitional or turbulent flow at the lowestpossible rotational speeds of the tool, and with the simplest toolgeometry.

It is therefore an aspect of the invention to teach a pin geometry thatwill result in transitional or turbulent flow of the workpiece material.FIG. 8 is provided as a profile view of a pin having physicaldeformations that are designed to obtain at least some transitional orturbulent flow of the workpiece material around the pin. As shown, thepin 120 is covered by a plurality of dimples 122, much like the dimplesof a golf ball. The number, size, and depth of the dimples 122 will needto be varied in order to obtain the desired flow characteristics for theworkpiece material.

Similarly, FIG. 9A is provided as an end view of a pin 126 and shoulder128 that is designed to generate transitional or turbulent flow aroundthe pin. The pin 126 is shown having a single flat 130 on a sidethereof. It is envisioned that the total number and the width of theflats 130 can be adjusted to obtain the desired flow characteristics ofthe workpiece. It is also envisioned that instead of a flat surface, thesurface irregularity will extend the length of the pin, and may not be auniform surface. For example, FIG. 9B shows another end view of a pin132 and shoulder 134, where a surface irregularity 136 is not flat.

Another aspect of the invention also related to obtaining transitionalor turbulent flow around the pin is shown in FIG. 10. FIG. 10 is aprofile view of a tool 140, where the pin 142 is disposed parallel tobut no longer concentric with a lengthwise axis 144 of the tool. The pin142 is now offset, thereby creating a cam configuration that is designedto generate transitional and turbulent flow in the workpiece material.It is noted that the degree of offset is exaggerated for illustrationpurposes only. The actual offset will depend upon the tool and theworkpiece characteristics.

It is also envisioned that many useful pin geometries and tools can beadapted in accordance with the principles of the present invention. Forexample, tools having pins of adjustable length can provide manybenefits. The tools must be modified to reduce stress risers, eithercoated on the shoulder and pin with superabrasive materials ormanufactured from solid superabrasive materials, and utilize thermalmanagement techniques as taught in the present invention.

A last aspect of the invention is the subject of pressing a tool to anear net shape. Near net refers to a tool that after pressing requiresvery little finishing to obtain the final product. In the presentlypreferred embodiment, the pin, shoulder, integral pin and shoulder, andpin with reinforcement are pressed to near net shape.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those-skilled in the art without departing from the spiritand scope of the present invention. The appended claims are intended tocover such modifications and arrangements.

1. A friction stir welding tool that is capable of functionally frictionstir welding a workpiece that is comprised of metal matrix compositesMMCs), ferrous alloys, non-ferrous alloys, or superalloys, said frictionstir welding tool comprising: a friction stir welding tool having ashank, a shoulder and a pin, wherein the pin includes at least onefeature designed to increase turbulent flow in the workpiece material;and a superabrasive material disposed on at least a portion of theshoulder and the pin; and wherein the friction stir welding tool iscapable of functionally friction stir welding MMCs, ferrous alloys,non-ferrous alloys, and superalloys.
 2. The friction stir welding toolas defined in claim 1, wherein the at least one feature on the pin iscomprised of a plurality of dimples distributed thereon.
 3. The frictionstir welding tool as defined in claim 1 wherein the pin is a replaceablepin to thereby enable different features on the pin to be disposed inthe friction stir welding tool to thereby increase turbulent flow in theworkpiece.
 4. The friction stir welding tool as defined in claim 3wherein the shoulder is a replaceable shoulder.